One of my favorite code sections, which basically says RTFM...
National Electrical Code 2014
Article 110 Requirements for Electrical Installations
110.3 Examination, Identification, Installation and Use of Equipment.
(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
If the installation manual says that 40 ampere overcurrent protection is required, then 40 ampere overcurrent protection is all that's required.
If you read further through the code, you'll get to article 422. There you'll find 422.10, which says.
Article 422 Appliances
422.10 Branch Circuit Rating.
(A) Individual Circuits. ...
... Branch circuits and branch circuit conductors for household ranges and cooking appliances shall be permitted to be in accordance with Table 220.55 and shall be sized in accordance with 210.19(A)(3).
This says that instead of simply using the rated values directly, you can use values found in these other code sections instead. If you look at Table 220.55, you'll find that if you only have 1 range, you can use 8 kW instead of the rated 12 kW.
Article 220 Branch Circuit, Feeder, and Service Calculations
III. Feeder and Service Load Calculations
220.55 Electric Ranges and Other Cooking Appliances — Dwelling Unit(s). The load for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances individually rated in excess of 1 3/4 kW shall be permitted to be calculated in accordance with Table 220.55. Kilovolt-amperes(kVA) shall be considered equivalent to kilowatts (kW) for loads calculated under this section.
This means that you'd only have a demand of 33.33 amperes.
8000 W / 240 V = 33.33 A
However, we also have to take 210.19(A)(3) into account, which says...
Article 210 Branch Circuits
II. Branch-Circuit Ratings
210.19 Conductors -- Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(3) Household Ranges and Cooking Appliances. Branch circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8 3/4 kW or more rating, the minimum branch-circuit rating shall be 40 amperes.
This means that since you have a range with a rating more than 8 3/4 kW, the minimum circuit rating is 40 amperes. So despite the fact that you used Table 220.55 to calculate a rating of 33.33 amperes, the circuit cannot be less than 40 amperes. Therefore, you'll install conductors rated for 40 amperes, and a 40 ampere overcurrent device.